Flyfan...a good post, i can't disagree too much with your points...
Could a 737 replacement be shrunk/stetched to meet the 737/757 market and would it be sufficiently more efficient than an A220 or A319/A320/A321 to be viable?
The 787 takes care of the 250+ pax market
The 777X fills out the top end of the long haul market pretty well, and the end of the A380 programme shows that anything bigger isnt needed..
But strapping lots of go faster/more efficient goodies onto a 1960s airframe is no longer an option!